In co-pending application Ser. No. 708,307 filed July 26, 1976, there is disclosed a way of motion analysis employing a suitable light source such as a laser beam reflecting from retroreflectors positioned on an element whose motion is to be analyzed. For example, two or more retroreflectors may be positioned on a rotating blade or other element in motion, and a laser beam directed to such retroreflectors with the reflected beams creating interference patterns on a detector. The system according to that invention is effective to analyze vibration or similar motion of turbine blades or the like and to analyze the motion of a more distant object, in each case to determine the amplitude of vibration, the speed of motion or other factors of motion being studied. It is now believed that the invention described in that co-pending patent application is the most sophisticated way of motion analysis now available for such study.
The invention of that prior application does not, however, in and of itself designate the direction of motion. It is capable of determining the amplitude and period of vibration but standing alone it does not determine the phase of such vibration. When the prior system is applied to distant moving objects such as, for example, aircraft or the like, it can determine when and how fast an airplane is tipping or turning, but does not in and of itself determine whether the object is tipping or turning toward or away from the source of a laser beam. Such directional analysis heretofore has required secondary information or other means such as, for example, computerized problem-solving. Similarly, a distant object such as a boat or ship whose linear motion may be relatively inconsequential can also be monitored with, again, similar limitations to the measurement of amplitude, frequency, etc. (yaw, pitch and roll) but not the direction thereof. Accordingly, there is a need for a way to pursue motion analysis which, also, tells the direction of such motion.
According to the present invention a way has now been found for motion analysis including determination of the direction of motion toward or away from the source of study. Light or similar radiation from a coherent source is formed into separable beams which are separately reflected, and the carrier frequency of one of them is modified. For example, coherent light, such as, for example, a laser beam is directed to two or more motion monitoring elements such as retroreflectors, and the reflected beams are causing to beat on one another. An increasing beat rate designates motion in one direction to shorten a light path, and a decreasing beat rate designates lengthening the light path.
According to one embodiment of the invention, the source of coherent radiation such as, for example, a laser beam is directed to an element whose motion is to be studied. Part of the beam is passed through a quarter wave plate then a plane polarizer to convert unpolarized light to plane polarized light. This beam is directed to a retroreflector mounted on an element whose motion is to be studied. After reflection from such retroreflector the beam is again passed through a quarter wave plate to convert the plane polarized light to circularly polarized light, for instance right hand circularly polarized light. The beam is passed through another quarter wave plate to return the circular polarization to linear. The return beam from other retroreflector is converted to left hand circular polarized radiation and re-converted to linear polarized radiation of the opposite direction from the first beam.
One of the two beams of linearly polarized radiation is frequency shifted so that it has a carrier frequency slightly different from the other beam. According to one embodiment as shown in the drawings and as described, an acousto-optic modulator shifts the frequency of one beam. In another embodiment, not shown, a beam may be doppler shifted by reflection from a moving body or by other frequency shifting means.
After the two reflected beams have been circularly polarized and converted back to linear polarization, and after one of the beams has been frequency shifted, then made to have the same polarization as the other beam, then the two beams are directed to the face of a photodetector. The two different frequencies cause beats on the face of the photodetector, and a change in the frequency of the beats designates that the total path from the laser source to the retroreflectors and back to the photodetector is changing. An increase designates that the higher frequency beam is travelling through a decreasing total path length compared with its companion beam, and a decrease in beat frequency designates that the higher frequency beam is going through an increasing path length.
Accordingly, the beat frequency in and of itself designates three factors of motion. In the case of vibration, the period between maximum or minimum in beat frequency corresponds to the period of vibration of the element being observed. Next the beat frequency designates the velocity of relative motion between the two retroreflectors; in the case of vibrational motion this velocity of relative motion also designates amplitude. The third motion factor determined by the beat frequency is the direction of motion of the one element relative to the other to lengthen or shorten the total beam path of the higher frequency beam.